# Copyright 2016 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""Caching utilities."""

import inspect
import weakref


# TODO(mdan): Add a garbage collection hook for cleaning up modules.
class _TransformedFnCache(object):
    """Generic hierarchical cache for transformed functions.

    The keys are soft references (i.e. they are discarded when the key is
    destroyed) created from the source function by `_get_key`. The subkeys are
    strong references and can be any value. Typically they identify different
    kinds of transformation.
    """

    __slots__ = ("_cache",)

    def __init__(self):
        self._cache = weakref.WeakKeyDictionary()

    def _get_key(self, entity):
        raise NotImplementedError("subclasses must override")

    def has(self, entity, subkey):
        key = self._get_key(entity)
        parent = self._cache.get(key, None)
        if parent is None:
            return False
        return subkey in parent

    def __getitem__(self, entity):
        key = self._get_key(entity)
        parent = self._cache.get(key, None)
        if parent is None:
            # The bucket is initialized to support this usage:
            #   cache[key][subkey] = value
            self._cache[key] = parent = {}
        return parent

    def __len__(self):
        return len(self._cache)


class CodeObjectCache(_TransformedFnCache):
    """A function cache based on code objects.

    Code objects are good proxies for the source code of a function.

    This cache efficiently handles functions that share code objects, such as
    functions defined in a loop, bound methods, etc.

    The cache falls back to the function object, if it doesn't have a code object.
    """

    def _get_key(self, entity):
        if hasattr(entity, "__code__"):
            return entity.__code__
        else:
            return entity


class UnboundInstanceCache(_TransformedFnCache):
    """A function cache based on unbound function objects.

    Using the function for the cache key allows efficient handling of object
    methods.

    Unlike the _CodeObjectCache, this discriminates between different functions
    even if they have the same code. This is needed for decorators that may
    masquerade as another function.
    """

    def _get_key(self, entity):
        if inspect.ismethod(entity):
            return entity.__func__
        return entity
